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Some gauge configurations are available for use on request
(see a list).
Please request gauge configurations from ilgti at theory
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A brief history of strong interactions

Atoms

Atoms as an indivisible component of matter were hypothesized
by many thinkers through history. Converting these philosophical
speculations into today's science required almost 300 years of
quantitative work; eventually achieving levels of precision which were
undreamt of in previous history.

The modern notion of the
atom follows from fundamental discoveries in chemistry. The
foremost among these were the law of conservation of mass (1789) by Lavoisier,
and the law of stoichiometry (1799) by Proust.
By putting these together with the gas laws of Boyle (1622) and Charles (1788),
Dalton reached the
conclusion (1803) that the atom was the smallest unit of matter which retained
the chemical properties of larger amounts of the same kind of matter.

This notion was sharpened through the following couple of centuries. When
the differences between chemical elements and chemical compounds became
clear, then the name atom began to be used only for the smallest unit of
an element which retained the chemical properties of the larger amounts of
that element. For compounds the smallest element is called a molecule.
The experimental methods of the 18th and 19th centuries could never
see individual atoms. While the quantitative usefulness of the notion of
atoms was clear, the fact that they could not be seen led to philosophical
discussions about whether atoms "really" existed. Eventually,
in 1905, Einstein demonstrated the reality of atoms through his analysis
of Brownian
motion.

Meanwhile, with improvements in the sensitivity of measurements which began
in the closing years of the 19th century, it became possible
to make measurements at the atomic scale. Brownian motion was just the first
of these observations. The most important was the count of the number
of molecules in one gram mole of a chemical compound, a number which is now
called Avogadro's
Number. This is a 24 digit number, whose value is approximately
6.022 141×1023. From a knowledge of Avogadro's number one
can find the typical weight of a molecule or atom. It is equal to

the atomic or molecular weight × 1/(6.022 141×1023) gms.

Soon experiments had
measured the physical size of an atom; it turned out to be around
10-9 m (called a nano-meter).

In the early years of the 20th century it was found that atoms could be
divided: they contained electrons. Since atoms are not electrically charged
whereas electrons are negatively charged, it followed that the atom also
contained positively charged particles. Rutherford performed a series of
experiments which showed that the positive charge resided in an atomic
nucleus that was a thousand times smaller than the atom, and contained
about 99.9% of its mass. This is the modern picture of an atom: a massive
positively charged nucleus surrounded by a cloud of almost massless electrons
distributed over distances 1000 times the size of the nucleus. The atom is
held together by electromagnetic forces. These electromagnetic forces are
also the main forces in chemistry.

Strong interactions

The story of strong interactions is relatively brief: it covers only
a hundred years; but it is full of many deeply mysterious phenomena,
each of which had to be solved using methods which were developed
specially for that problem. As a result, this is the one field which
has gathered the maximum number of Nobel
prizes: the most recent in 2008.

Since the atom is divisible, it is not elementary. Among the elementary
particles one had to count the negatively charged electron. It became clear
that the nucleus of hydrogen was another. This was called the proton. Electrical
forces bound electrons and nuclei together into atoms, since they had opposite
charges. How could the nucleus, made entirely of positive charges be bound
together? Positive charges would repel. Since they are bound, there must be
a new force involved: this is the strong nuclear force.

The new science of nuclear physics involved itself with the study of
the strong force. The electrically neutral neutron was discovered to
be another elementary particle which is found in the nucleus. Just
as the photon is the particle involved in the transmission of the
electromagnetic forces, a different particle was found to be involved
in the transmission of the strong nuclear force; it was given the name
"pion". By the 1940s the full theory of the nucleus seemed
to be within reach.

In the 1950's there were a series of new discoveries which complicated
everything again. Particle accelerators were constructed to smash together
particles at high speeds. In the debris of these collisions many new
particles were found. Most of these particles had strong charge. There
seemed to be a large periodic table of strongly interacting particles:
the hadrons. Was there a "periodic table" of hadrons? What
principles would underlie such a table?

When a periodic table was constructed, and new particles were predicted
and found, then the principles became clear. The hadrons were not
elementary at all: for 50 years people had been looking in the wrong
place for the elements of matter. The elementary particles were fewer
in number and were given the name of quarks. Strong forces bound the
quarks into hadrons and the strong nuclear forces were but a shadow of
these forces. After twenty years of development it became clear that
these quarks were real, and there were new particles which carried the
strong force: called gluons. A theory of the strong interactions was
developed and tested. This is called Quantum ChromoDynamics (QCD).

Quantum ChromoDynamics

QCD is a theory of quarks and gluons. It is the most mathematically
intractable among all fundamental theories that we have. It has been
tested in high energy collisions, where the calculations become relatively
simple. However, all other aspects, including the existence of hadrons
and the emergence of nuclear physics, have not yet been derived
from the theory.

The main method for systematic and accurate calculation in a field
theory is "perturbative": a power series expansion in the
strength of the interaction. If the interaction strength is called
α, then one can try to write any quantity as

(#) + (#) α + (#) α2 + (#) α3 + ...

where each of the numbers (#) can be calculated from the theory. When
α is small, as it is for electromagnetic and weak interactions,
the series of terms above is very well approximated by the first few
terms. As a result, this method works well. Strong interactions are
involve large α, as a result of which the method can fail.

For reasons which are intimately connected with the nature of quantum
theories, the strong coupling strength becomes effectively weak for
high energy processes. This is called asymptotic freedom. As a result,
the series above can be approximated by a small number of terms, and
predictions for high energy collisions can be made with accuracy. QCD
has been tested and found to be correct in this domain.

The only other systematic quantitative method is one developed by Kenneth
Wilson called lattice gauge theory. In this method one uses supercomputers
to compute the predictions of a theory without resorting to a series
expansion as above. For QCD this is the only method available for most
calculations. Using this method it is predicted that matter above a
temperature of about 1 trillion degrees or density of a few times larger
than nuclear density undergo a transition to a new form called the quark
gluon plasma.

This is not the only new phase of matter that is predicted by QCD. At
small temperatures but extremely high densities there are superconducting
phases. These are probably the highest temperatures at which matter
is superconducting. These extreme phases of matter are currently being
investigated on supercomputers, probed in experiments and searched for
in the universe at large.

Our calculations in Mumbai have also predicted a temperature and a density
at which there is no distinction between normal nuclear matter and the quark
gluon plasma. This prediction is under experimental test at the nuclear
accelerator called the RHIC in the
Brookhaven National Laboratory in USA.